1. Field of the Invention
The present invention relates to SAW filter devices used as band-pass filters for mobile telephones, and, more particularly, to a SAW filter device in which a plurality of one-port SAW resonators are connected.
2. Description of the Related Art
Currently, various surface acoustic wave filters are used as band-pass filters included in RF stages in communication apparatuses, such as mobile telephones. A known surface acoustic wave filter is disclosed in Japanese Unexamined Patent Application Publication No. 9-121136.
In a surface acoustic wave filter disclosed in Japanese Unexamined Patent Application Publication No. 9-121136, a series arm surface acoustic wave resonator, an output electrode, a parallel arm surface acoustic wave resonator, and a ground electrode are disposed on a 41°-rotated Y-cut X-propagation LiNbO3 substrate. The series arm surface acoustic wave resonator includes IDTs, one of which is connected to the output electrode. The parallel arm surface acoustic wave resonator includes IDTs, one of which is connected to the output electrode and the other one of which is connected to the ground electrode. These IDTs are formed from metal films made of Al or an Al alloy, and the thickness of the metal films ranges from about 2.5% to about 7.5% of the electrode cycle of the IDTs included in the parallel arm surface acoustic wave resonator. Consequently, a frequency characteristic having a wide and flat pass band with no spurious components is obtained.
“Hyomenha Debaisu To Sono Oyo” (Surface Acoustic Wave Device and Application Thereof), edited by Electronic Materials Manufacturers Association of Japan, published by The Nikkan Kogyo Shimbun, Ltd., Dec. 25, 1978, First Edition, pp. 17 and FIG. 1.7 discloses changes in phase velocity and electromechanical coupling coefficient of a Rayleigh wave and a leaky surface acoustic wave which propagate on a LiNbO3 substrate. The changes are caused by the cutting angle of the LiNbO3 substrate. That is, “Hyomenha Debaisu To Sono Oyo” (Surface Acoustic Wave Device and Application Thereof), edited by Electronic Materials Manufacturers Association of Japan, published by The Nikkan Kogyo Shimbun, Ltd., Dec. 25, 1978, First Edition, pp. 17 and FIG. 1.7 describes the fact that, with a 41°-rotated Y-cut X-propagation LiNbO3 substrate whose cutting angle is 41°, the electromechanical coupling coefficient of a leaky surface acoustic wave has a large value, but a Rayleigh wave exhibits piezoelectricity, and, with a cutting angle of 60° to 70°, the electromechanical coupling coefficient of a leaky surface acoustic wave has a value smaller than that obtained in the case of the cutting angle of 41°, but a Rayleigh wave does not exhibit piezoelectricity.
FIG. 1.7 in “Hyomenha Debaisu To Sono Oyo” (Surface Acoustic Wave Device and Application Thereof), edited by Electronic Materials Manufacturers Association of Japan, published by The Nikkan Kogyo Shimbun, Ltd., Dec. 25, 1978, First Edition, pp. 17, describes the fact that the propagation velocity ratio of a Rayleigh wave to a leaky surface acoustic wave is about 0.8 when a cutting angle ranges from about 41° to about 60°.
In Japanese Unexamined Patent Application Publication No. 9-121136, the IDTs having the above-described predetermined film thickness are disposed on the 41°-rotated Y-cut X-propagation LiNbO3 substrate, whereby a wide pass band and the reduced spurious component are achieved. However, if a filter device having a trap band and a pass band lower than the trap band is created using this LiNbO3 substrate, a wide trap band can be obtained, but an insertion loss is increased in the pass band lower than the trap band and in a high frequency end portion of the pass band.
That is, as described in “Hyomenha Debaisu To Sono Oyo” (Surface Acoustic Wave Device and Application Thereof), edited by Electronic Materials Manufacturers Association of Japan, published by The Nikkan Kogyo Shimbun, Ltd., Dec. 25, 1978, First Edition, pp. 17 and FIG. 1.7, with the 41° to 60°-rotated Y-cut X-propagation LiNbO3 substrate, the propagation velocity ratio of a Rayleigh wave to a leaky surface acoustic wave is about 0.8. Accordingly, if a filter device having a pass band lower than a trap band is created using this LiNbO3 substrate, a spurious component is generated under the influence of a Rayleigh wave at a frequency that is about 0.8 times a trap frequency. This causes a deterioration of an insertion loss in the pass band.